ORIGINAL: Charley
Not really "overcooled." The engineers who decide on the size and shape of the cooling fins are taking the heat transfer rates from inside parts to the outside fins into consideration. In fact, on airplanes, we sometime have to put baffles inside the cowls in order to direct enough cooling air past the cylinder fins. Look at some heli engines. They have large "heat-sink" heads to make up for lower rates of air flow over the fins.
CR
I'd like to believe that model engines have this much engineering effort put into them, but I doubt it. Most if it is based on past experience or emprical testing. Air cooling is a tricky thing. Glow engines are not too critical of cooling while gasoline is more critical. Even though the a model airplane engine is out in the breeze the airflow around the cylinder isn't particularly effective. A proper cowling and ducting would have much less drag and only a very small cooling airflow would cool it just as well.
Automotive engines have a set operating point due to the water cooling system and crankcase oil. Running too hot will boil the coolant, running too cool is not efficient, and causes wear among other things. Two stroke air cooled engines just need overheating prevention.